1. Field of the Invention
The present invention relates to an arc tube for a discharge lamp unit, having a closed glass bulb held between pinch seal portions located at opposite ends of the closed glass bulb, and a pair of electrodes provided in the closed glass bulb so as to be opposite to each other. Particularly it relates to a mercury-free arc tube for a discharge lamp unit, having a closed glass bulb not containing mercury but containing main light emitting metal halide and starting rare gas enclosed in the closed glass bulb.
2. Description of the Related Art
FIG. 4 shows a discharge bulb which is a conventional discharge lamp unit used as a light source of an automotive lamp. The discharge bulb has a structure in which an arc tube 2 having a closed glass bulb 2a as a light emitting portion is integrated with an electrically insulating plug body 1 made of a synthetic resin. A rear end portion of the arc tube 2 is gripped by a metal support member 8 fixed to the electrically insulating plug body 1. A front end portion of the arc tube 2 is supported by a metal lead support 9 which serves also as a current conduction path extended out from the electrically insulating plug body 1.
The arc tube 2 has a structure in which main light emitting metal halide, buffer mercury and starting rare gas are enclosed in the closed glass bulb 2a which is held between pinch seal portions 2b and 2b located at opposite ends of the close glass bulb 2a and which is provided with a pair of electrodes 3 and 3 so as to be opposite to each other. Light is emitted on the basis of arc generated by electric discharge between the pair of electrodes 3 and 3. The discharge bulb is superior to that of an incandescent bulb because a large quantity of emitted light, a long lifetime, etc. can be achieved by the discharge bulb. For this reason, nowadays there is a tendency for the discharge bulb to be used as a light source for a head lamp or a fog lamp.
The reference numeral 4 designates a lead wire led out from each pinch seal portion 2b. The reference numeral 5 designates a sheet of molybdenum foil for connecting the lead wire 4 to a corresponding tungsten electrode 3. Further, ultraviolet-shielding shroud glass 6 is integrally welded to the arc tube 2 to thereby form a structure in which the closed glass bulb 2a is surrounded by a closed space formed by the shroud glass 6. Hence, the closed glass bulb 2a is kept at a high temperature while ultraviolet rays in a wavelength range harmful to the human body are cut off from light emitted from the arc tube 2.
Mercury enclosed in the conventional closed glass bulb 2a is a substance toxic to the environment. In response to the social needs of reducing the cause of global environmental pollution as much as possible, it is desirable that a mercury-free arc tube not containing mercury, which is a substance toxic to the environmental, is developed.
The following findings have been obtained in the process of research and development on a mercury-free arc tube not containing mercury.
Mercury acts mainly as buffer substance for keeping the tube voltage constant to reduce the amount of electrons colliding with electrodes to thereby buffer the damage of is the electrodes. Mercury acts also as a light emitting substance for emitting white light. For this reason, if mercury was removed from the substances enclosed in the closed glass bulb, the following changes (problems) occurred in the characteristic as the arc tube.
Firstly, the tube voltage is reduced. That is, because the tube electric power required for electric discharge cannot be obtained, it is necessary to increase the tube current in order to increase the tube electric power. Hence, the load imposed on the electrodes increases, so that luminous efficacy is lowered. Moreover, the amount of heat generated in ballast increases because of the increase of the tube current, so that system efficiency is lowered.
Secondly, the rising edge of luminous flux is delayed. That is, a sufficient quantity of emitted light cannot be obtained at an initial stage of operation (electric discharge) because mercury high in vapor pressure is absent.
Thirdly, luminous flux in a visible region is reduced because light emitted due to mercury is absent.
Fourthly, the color of emitted light is different (reddish) from the color of light emitted from the conventional mercury-containing arc tube.
Fifthly, luminous intensity distribution control can be hardly performed because the curvature of arc generated by electric discharge between the electrodes is large.
The spectral characteristic of a mercury-free arc tube having main light emitting metal halide (NaI and ScI3) and a rare gas enclosed in a closed glass bulb was as represented by the solid line in FIG. 5. It has been found that the mercury-free arc tube generally exhibits substantially the same spectral characteristic as that of the conventional mercury-containing arc tube except that the intensity of light particularly in wavelength ranges near to 435 nm and 546 nm in the mercury-free arc tube is lower (lower in peak) by the quantity represented by the broken line in FIG. 5 than that in the conventional mercury-containing arc tube.
Therefore, experiments have been performed to examine whether characteristics approaching the characteristics of the conventional mercury-containing arc tube can be obtained or not. The experiments are directed to a selection of metal halide to be enclosed, which has a buffer function and is effective in increasing the intensity of light in wavelength ranges near to 435 nm and 546 nm in spectral characteristic. Such metal halide should be selected as a substitute of mercury having a buffer function and be enclosed while the charged pressure of a rare gas enclosed in the closed glass bulb and a quantity and ratio of enclosure of the main light emitting metal halide are adjusted without changing the shape and dimensions of the conventional mercury-containing arc tube as possible. As a result, it has been confirmed that the characteristic can be improved. Thus, Japanese patent application No. 2001-286252 is proposed (applied).
After then, as a result of continuous experiments by the inventors, it is confirmed that there can be provided a mercury-free arc tube which has characteristics approaching the characteristics of a conventional mercury-containing arc tube even if buffer metal halide is not enclosed in the closed glass bulb while a total amount and ratio of the main light emitting metal halide and the charged pressure of starting rare gas enclosed in the closed glass bulb are adjusted. Thus, Japanese patent application No. 2002-243489 is proposed (applied).
The present invention is based on the problems in the conventional art and the inventor""s knowledge and an object thereof is to provide a mercury-free arc tube for a discharge lamp unit in which characteristics approaching the characteristics of the conventional arc tube can be obtained.
In order to achieve the object, according to a first aspect of the invention, there is provided a mercury-free arc tube, which is not containing mercury, for a discharge lamp unit, in which electrodes are opposite to each other and provided in a closed glass bulb held between pinch seal portions, and main light emitting metal halide is enclosed; wherein starting rare gas has a charged pressure set to be in a range of from 8 to 20 atmospheres.
Preferably, according to a second aspect of the invention, buffer metal halide is enclosed in the closed glass bulb together with the main light emitting metal halide and the starting rare gas.
(Operation) Substances used in the conventional case are used as the main light emitting metal halide and the starting rare gas. That is, sodium-scandium-based halide such as NaI and ScI3 is used as the former and Xe is used as the latter.
The buffer metal halide in place of mercury is at least one selected from the group consisting of halides of Al, Bi, Cr, Cs, Fe, Ga, In, Mg, Ni, Nd, Sb, Sn, Tb, Tl, Ti, Li and Zn. When the buffer metal halide is enclosed in the closed glass bulb, great reduction of the tube voltage caused by no mercury enclosed can be suppressed.
Particularly when the charged pressure of the starting rare gas is set at a pressure (of 8 to 20 atmospheres), which is higher than the charged pressure (of 3 to 6 atmospheres) in the conventional arc tube, the ratio at which electrons released from the electrodes at electric discharge collide with molecules of the rare gas is increased to raise the temperature of the inside of the closed glass bulb in operation (at electric discharge). Accordingly, the vapor pressure of the main light emitting metal halide (as for the second aspect of the invention, the main light emitting metal halide and the buffer metal halide) is increased to thereby increase the tube voltage. Hence, it is unnecessary to significantly increase the tube current, and there is no possibility that the load imposed on the electrodes may significantly increase because of the increase of the tube current. As a result, there is no fear that luminous efficacy may be significantly lowered, and there is no significant increase in the amount of heat generated in ballast because of the increase of the tube current, there is no significant lowering of system efficiency. That is, contribution to solution of the first problem (reduction of tube voltage) is achieved.
When at least one kind of metal halide which generates an emission color close to the emission color of mercury and which is selected from the group consisting of halides of Al, Bi, Cr, Cs, Fe, Ga, In, Mg, Ni, Nd, Sb, Sn, Tb, Tl, Ti, Li and Zn is enclosed in the closed glass bulb, the enclosure compensates for reduction in the quantity of emitted (white) light in a visible region and reduction in the luminous flux. Particularly because the charged pressure of the starting rare gas is high (8 to 20 atmospheres), the temperature of the inside of the closed glass bulb in operation (at electric discharge) is made high as described above. Hence, the vapor pressure of the buffer metal halide is increased and the intensity of light in wavelength ranges near 435 nm and/or 546 nm is increased, so that substantially the same quantity of emitted light as that of light emitted from the conventional arc tube can be obtained when the color of emitted light is white which is substantially the same as the color of light emitted from the conventional arc tube.
Further, as mentioned in the first aspect of the invention, even if the buffer metal halide is not enclosed, the vapor pressure of the main light emitting metal halide, whose quantity and/or ratio of enclosure is adjusted, is increased. Hence, substantially the same quantity of emitted light as that of light emitted from the conventional arc tube can be obtained when the color of emitted light is white which is substantially the same as the color of light emitted from the conventional arc tube.
That is, contribution to solution of the third and fourth problems (reduction in luminous flux and color of emitted light) is achieved.
Because the charged pressure of the starting rare gas is high, the temperature of the inside of the closed glass bulb in operation (at electric discharge) is made high as described above. Hence, the vapor pressure of the main light emitting metal halide (sodium-scandium-based halide such as NaI and ScI3) is increased, so that the luminous flux is increased. That is, contribution to solution of the third problem (reduction in luminous flux) is achieved.
When the charged pressure of the rare gas is high, the DC resistance component (impedance) at starting is increased. Hence, the consumed electric power is increased, so that the temperature of the closed glass bulb in operation (at electric discharge) rises rapidly. That is, contribution to solution of the second problem (rising edge of luminous flux) is achieved.
Moreover, the center temperature of the arc in the closed glass bulb rises, so that the center luminance of the arc increases. That is, contribution to solution of the third problem (reduction in luminous flux) is achieved.
According to a third object of the invention, there is provided a mercury-free arc tube for a discharge lamp unit, as described in the first or second aspect of the invention, in which a ratio (D2/D1) of an inner diameter D2 (mm) at a position near a tip end portion of the electrode to an inner diameter D1 (mm) at a position in the middle of the electrodes of the closed glass bulb is in a range of from 0.5 to 1.1, preferably, from 0.6 to 1.0.
(Operation) According to the experiments by the inventors, it is confirmed, as shown in FIG. 2, that the ratio (D2/D1) of an inner diameter D2 (mm) at a position near a tip end portion of the electrode to an inner diameter D1 (mm) at a position in the middle of the electrodes of the closed glass bulb affects a shape of an arc, stability of discharge, a devitrification phenomenon and re-firing voltage in the closed glass bulb. Further, it is confirmed that D2/D1 should be in a range of from 0.4 to 1.1 for appropriateness of the shape of the arc (linearity of the arc), that D2/D1 should be in a range of from 0.5 to 1.2 for the stability of discharge (stable discharge without blink), that D2/D1 should be 0.5 or more for avoiding the devitrification phenomenon of the closed glass bulb, and that D2/D1 should be 0.5 or more for appropriateness of re-firing voltage. Hence, it is preferable that D2/D1 should be in a range of from 0.5 to 1.1, more preferably in a range of from 0.6 to 1.0, in order to satisfy all conditions for the appropriate shape of the arc, the stability of discharge, the avoidance of the devitrification phenomenon and the appropriate refiring voltage.
According to a fourth aspect of the invention, the mercury-free arc tube for the discharge lamp unit, mentioned in one of the first to third aspects of the invention, is constituted such that the closed glass bulb has a maximum inner diameter in a range of from 2.0 to 3.5 mm; a distance between the electrodes is in a range of from 4.0 to 4.4 mm; a length of protrusion of the electrode into the closed glass bulb is in a range of from 1.0 to 2.0 mm; and a quantity of enclosure of the main light emitting metal halide is in a range of from 0.1 to 0.6 mg.
(Operation) The distance between the electrodes is set to be in a range of from 4.0 to 4.4 mm to satisfy ECE standards, like the conventional case. The length of protrusion of the electrode into the closed glass bulb is set at a dimension ranging from 1.0 to 2.0 mm but slightly smaller than the conventional length (1.8 to 2.0 mm) of protrusion. The axial length of the inside of the closed glass bulb is formed to be not larger than the axial length of the inside of the conventional closed glass bulb. In addition, the maximum inner diameter of the closed glass bulb is set to be in a range of from 2.0 to 3.5 mm and slightly smaller than the maximum inner diameter of the conventional closed glass bulb. Hence, the vapor pressure of the main light emitting metal halide in the closed glass bulb is made high in accordance with reduction in volume of the closed glass bulb, so that the luminous flux increases. Moreover, the heat capacity of the closed glass bulb is made low in accordance with reduction in volume of the closed glass bulb, so that the temperature of the inside of the closed glass bulb at starting rises rapidly. That is, contribution to solution of the second problem (rising edge of luminous flux) is achieved. Incidentally, the amount of enclosure of the main light emitting metal halide is preferably in a range of from 0.1 to 0.6 mg having enlarged upper and lower limits compared with the range of from 0.2 to 0.4 mg as the amount of enclosure in the conventional mercury-containing arc tube.
Moreover, the inside of the closed glass bulb is narrowed in the direction of the radius thereof so that the curvature of the inner circumferential surface of the closed glass bulb surrounding the inter-electrode region in which an arc is generated is reduced. In accordance with this reduction, the curvature of the arc is reduced and the arc size is reduced. As a result, luminous intensity distribution control of the arc can be made easily. That is, contribution to solution of the fifth problem (curvature of arc) is achieved.
While the distance between the electrodes is set to be in a range of from 4.0 to 4.4 mm to satisfy ECE standards, the length of protrusion of each of the electrodes into the closed glass bulb is set to be in a range of from 1.0 to 2.0 mm and slightly smaller than the conventional length (1.8 to 2.0 mm) of protrusion. Hence, condensation of the main light emitting metal halide (NaI, ScI3) onto the base of each of the electrodes is avoided so that a predetermined chromaticity is obtained. That is, contribution to solution of the fourth problem (color of emitted light) is achieved.
In this manner, a mercury-free arc tube having the shape and dimensions similar to those of the conventional mercury-containing arc tube and having characteristics substantially equivalent to the characteristics of the conventional mercury-containing arc tube can be produced.
According to a fifth aspect of the invention, in the mercury-free arc tube for the discharge lamp unit stated in one of the first to fourth aspects of the invention, cylindrical shroud glass is integrally welded to the arc tube to thereby form a closed space surrounding the closed glass bulb, and an inert gas with a pressure not higher than 1 atmosphere is enclosed in the closed space.
(Operation) Because the molecular density of the inert gas enclosed in the closed space surrounding the closed glass bulb is low, heat transfer between the closed glass bulb and the shroud glass through the closed space is suppressed. For this reason, heat in the closed glass bulb hardly goes to the outside, so that the temperature of the inside of the closed glass bulb is kept high. Hence, the vapor pressures of the main light emitting metal halide, the buffer metal halide and the rare gas in the closed glass bulb are made high, so that the tube voltage increases to thereby improve the first problem (reduction of tube voltage). Moreover, the vapor pressure of the main light emitting metal halide in the closed glass bulb is made high, so that the luminous flux increases to thereby improve the third problem (reduction in luminous flux). Moreover, at an initial stage of operation (electric discharge), the temperature of the inside of the closed glass bulb is raised rapidly to thereby make the vapor pressure of the main light emitting metal halide high. That is, the second problem (rising edge of luminous flux) is improved.